Radio transmitters and/or radio receivers may implement filtering in order to co-exist with other users and systems occupying neighboring channels and frequency bands. Moreover, in some radios a plurality of band selection filters may be used for each receive band and/or transmission band. When devices support several bands, multiple filters may be used for each band. However, these filters may be implemented with bulky and costly technologies not integrated with the rest of radio.
Although some cellular standards currently support about 4 or more bands, future cellular standards may support additional bands (for example, up to and exceeding 40 bands). In the case of a transmitter, the corresponding filters may be configured to primarily transmit noise that is outside of the transmit band. There is, however, a strict requirement with cellular radios regarding how much noise a transmitter can radiate inside the transmit band, so tunable narrowband filters may comply with that requirement. In the case of a receiver, a filter may also be used for channel selection inside a receive frequency band. This filter may be located at baseband, so only one filter is needed in a radio. However, some radios may not have traditional baseband processing, so any noise filtering may be done at the radio frequency front end. For example, N-path filter or trans-impedance filter may be used to provide channel selection at radio frequencies rather than at baseband. These filters may introduce low impedance for the interfering signals and high impedance for the desired signal. When this kind of impedance load is driven from a high impedance source, the interfering signals are attenuated.